The invention relates to a torque responsive release clutch mechanism intended for a pneumatic power nutrunner. In particular, the invention concerns a clutch mechanism comprising a driving clutch half connected to the nutrunner motor and being formed with pockets for two or more torque transferring balls, an axially acting bias spring, and an axially displaceable thrust element for transferring the load of the bias spring to the balls. The clutch mechanism also comprises a driven clutch half connected to the output shaft of the nutrunner and provided with cam surfaces for cooperation with the torque transferring balls.
A clutch mechanism of the above described type is previously disclosed in U.S. Pat. No. 5,129,465.
A problem concerned with clutches of the above type refers to the reengagement related torque impacts generated after release of the clutch at tightening of stiff screw joints, i.e. joints with a steep torque growth per unit angle of rotation. The reason is that when the desired maximum torque level is reached and the balls of the clutch pass over the top crests of the cam surfaces, the rotation speed is still high enough to cause the cam surfaces to reengage with the balls once or twice or more before the motor and the driving clutch half have stopped rotating, despite a timely motive air shut-off. Such torque impacts cause an undesirable overshoot of the installed torque beyond the desired torque level.
One way of solving this problem is to employ means for separating completely the clutch parts immediately after the clutch is released, thereby preventing the clutch from reengaging and generating undesirable torque impacts.
Examples of power nutrunners having clutch mechanisms provided with pneumatically actuated clutch separating means are disclosed in U.S. Pat. Nos. 2,683,512 and 3,487,901. Both of these prior art devices comprise pneumatic piston cylinder means associated with the driving clutch half and arranged to prolong the axial displacement of the driving clutch half after the attainment of the release point of the clutch. A feature common to these two devices is that the torque transferring engagement between the driving and driven clutch halves is determined by the air pressure acting on the piston-cylinder means. This makes the release torque level dependent on the air pressure, which is sometimes a drawback. Another feature common to these known devices is that the cylinder of the piston-cylinder means is formed as a nonrotating part of the tool housing and that the pressure air supply thereto requires extra space for internal passages through the tool housing or for external piping.
For cooperation with the nonrotating cylinder, the device shown in U.S. Pat. No. 2,683,512 comprises a rather primitive piston arrangement in which the piston forms a one-piece part of the driving clutch half. The disadvantages of this known clutch design are the occurence of friction forces and an undesirable wear of the piston and cylinder surfaces, which results in a poor function and a short service life of the clutch mechanism.
In the device shown in U.S. Pat. No. 3,487,901, the piston is rotatively journalled relative to the driving clutch half. This results in a more favourable interaction between the piston and the immovable cylinder. However, it also results in a rather complicated nutrunner design including extra ball bearings between the driving clutch half and the piston.